1. Field of the Invention
The invention relates generally to cancer and more specifically to agents that affect the Notch pathway.
2. Background Information
Notch genes encode evolutionarily conserved, large, single pass transmembrane proteins that regulate cell fate determination. Work in Drosophila, Caenorhabditis elegans and mammalian cell culture has shown that Notch acts as receptors for the DSL (Delta, Serrate, Lag-2) family of ligands and signal through two downstream pathways. One of these is via the CSL (CBF1, Suppressor of Hairless, Lag-1) family of transcription factors and the other via the cytoplasmic adapter protein Deltex. In mammals, the Notch signaling pathway includes four receptors (Notch 1-4) and five ligands (Delta-like 1, 3 and 4 & Jagged 1 and 2). Mutations in these genes can result in dramatic developmental effects in humans, implicating Notch signaling in several inherited diseases (e.g., Cerebral Autosomal Dominant Arteriopathy with Subcortical Infarcts and Leukaencephalopathy (CADASIL), Alagille syndrome and spondylocostal dysostosis (SCDO)), and cancers (e.g., leukemia, skin, cervical, lung cancer, prostate, neuroblastomas, and breast cancer).
Negative regulators of Notch signaling include Numb, SEL-10 and Su(dx). In contrast, Sanpodo, Neuralized, Mind bomb, LNX, Siah1 and Mdm2, are positive regulators of the pathway. Numb is a cytoplasmic adaptor protein containing a PTB domain, which acts as a negative regulator of Notch through its interaction with the intracellular domain. Numb-Notch association promotes ubiquitination of Notch1. It has been demonstrated that Numb interacts with the cytosolic HECT domain of E3 ligase Itch and that Numb and Itch act cooperatively to promote ubiquitination of Notch1. On the other hand, Numb may inhibit Notch signaling by altering the function of Sanpodo, which is a positive regulator of the pathway. Sandopo encodes four-pass transmembrane protein, which physically interact with the full length Notch receptor on cell surface. Numb physically interacts with Sanpodo and inhibits the membrane localization of Sanpodo preventing its association with Notch. The mammalian actin-associated protein, Tropomodulin, which controls actin filament length, is a homologue of Sanpodo. Numb and Sanpodo are key regulators of the Notch pathway during developmental events.
Suppressor of Deltex (Su(dx)) is another protein originally identified in Drosophila that acts as a negative regulator of Notch signaling pathway. Overexpression of Su(dx) can block endogenous Notch signaling leading to ectopic vein differentiation and loss of the wing margin. In contrast, downregulation of Su(dx) displays a wing vein gap similar to the Notch overexpression phenotype. Itch is the mouse homologue of Su(dx) and is so named because mice mutant display an itching behavior along with immunological defects. In contrast to SEL-10, Itch contains phospholipid binding motif that targets it to the plasma membrane, four WW-motifs and a HECT domain which functions as an E3 ubiquitin ligase. Itch binds to N-terminal portion of the NICD via its WW-motifs and promotes ubiquitination of NICD though its HECT domain. In mammalian cells this interaction downregulates Notch signaling.
The first indication that the Notch signaling pathway may play a role in the neoplastic development of the mammary gland came from a common insertion site within the Notch4 gene of the Mouse Mammary Tumor Virus (MMTV) in Czech II mice. This insertion results in the expression of a truncated transcript that encodes the intracellular domain of Notch4; expression of this protein activates the signaling pathway. The causal role of Notch signaling in tumor development was shown in transgenic mice expressing this protein specifically in the mammary gland. These mice display mammary tumors within 12 months. Furthermore, cell culture experiments demonstrated that overexpression of Notch1 or Notch4 intracellular domain transforms mouse mammary epithelial cell lines leading to anchorage independent growth in soft agar.
Notch signaling is aberrantly activated in a wide range of human breast cancers. This is most clearly demonstrated by the loss of Numb and the accumulation of the Notch1 intracellular domain (NICD), which is generated by cleavage of the full length protein during signaling and transduces the Notch signal, and the upregulation of the target genes Hes1 and Hey1. Furthermore, changes in pathway components may prove to be useful prognostic markers. For example, elevated transcript levels for Notch1 and the ligand Jagged1 correlate with poor prognosis, and proteosomal degradation of Numb is seen primarily in high grade tumors. Finally, the increase in Notch signaling plays an important role in the etiology of breast cancer, as inhibiting the pathway reverts the transformed phenotype of breast cancer cell lines and prevents the growth of primary tumor cells.
Although altered Notch signaling has been linked to human diseases, including cancer, evidence for a substantial involvement of Notch in human tumors has remained elusive. Mechanistically, Numb operates as an oncosuppressor, as its ectopic expression in Numb-negative, but not in Numb-positive, tumor cells inhibits proliferation. Increased Notch signaling is observed in Numb-negative tumors, but reverts to basal levels after enforced expression of Numb. Conversely, Numb silencing increases Notch signaling in normal breast cells and in Numb-positive breast tumors. Numb/Notch biological antagonism is relevant to the homeostasis of the normal mammary parenchyma. Thus, a need exists for a method of diagnosing and treating cancer caused by aberrant signaling of the Notch pathway.